This invention relates to a high resolution optical system used for inspecting and observing fine pattern defects, foreign matter, etc, which appear, for example, in manufacturing processes of semiconductor devices and flat panel displays. The invention also relates to a defect inspecting apparatus that uses such a high resolution optical system.
A conventional technique, which has provided a method and an apparatus for photographing the structures of fine lines in width using an optical microscope, is disclosed in Japanese Patent Laid-Open No. 7-128595. This technique is characterized by the use of light which is linearly polarized by a polarizer positioned at about 45° to the linear dimension of a sample. The optical delay axis of a ¼ wavelength plate placed between the polarizer and the sample is angled optimally (25° typically) to the main linear shape of the sample. This ¼ wavelength plate converts the linearly polarized light to elliptically polarized light, which is then applied to the sample. This elliptically polarized light, when reflected from the sample, has a phase difference. The reflected light passes through the ¼ wavelength plate again, then passes through a polarizer provided in a detecting light path. The light passing through the polarizer forms an image of the sample on a photoelectric conversion element. In such conventional apparatus, therefore, the phase difference caused by the sample is estimated beforehand, enabling the light set as elliptic polarized light to be converted to circularly polarized light after reflection from the sample.
In the method and apparatus for imaging structures of fine line width using an optical microscope as described, a polarizer is disposed in a lighting light path, and a linearly polarized light is passed through the polarizer. Then this linearly polarized light is converted to an elliptic polarized light through the ¼ wavelength plate before it is applied to the sample. In such an optical system, therefore, both 0-order diffracted light reflected from the sample and higher-order diffracted light become circularly polarized light, and the ratio between the amplitudes of the 0-order diffracted light effective for forming optical images and higher-order diffracted light is the same as that of the random polarized light (the amplitude of the 0-order diffracted light is larger than that of the higher-order diffracted light). Consequently, the 0-order diffracted light and the higher-order diffracted light interfere with each other, thereby degrading the resolution of the optical image of the sample, particularly because the amplitude of the higher-order diffracted light is small, and this causes the low frequency component to be increased.
If an image sensor is used for detecting images, the light intensity is adjusted so as to prevent saturation. For example, because a cyclical fine pattern increases the diffraction angle, the contrast between patterns is small, making detected images dark. If a pattern image is detected and this detected image is processed for defect inspection, the small difference in contrast between patterns, and the dark image causes lower detection sensitivity.